1. Field of the Invention
The invention relates generally to the field of oscillators or clocks used to control the timing of electronic devices. More particularly, the invention relates to the field of temperature stabilization techniques for clocks.
2. Background Art
Timing of operation of electronic devices, particularly digital devices, requires an accurate, frequency stable clock signal. Many such electronic devices are subjected to variations in ambient temperature during their operation. As is well known in the art, changes in ambient temperature affect the frequency of a typical oscillator, even highly stable quartz-crystal oscillators or clocks.
Various techniques have been developed to improve the frequency stability of a quartz clocks. One technique is to place the clock in a temperature controlled chamber. See, for example, U.S. Pat. Nos. 5,917,272, 5,729,181 and 5,180,942.
A second technique is to use a voltage controlled oscillator (VCO) which enables adjusting the frequency of the clock output to compensate for temperature variations. Typical VCO systems are described, for example, in U.S. Pat. Nos. 5,668,506 and 5,473,289. Generally speaking, VCO systems include a means for measuring temperature operatively coupled to the clock so that the clock frequency can be adjusted for temperature variations.
Another technique for clock frequency stabilization is using a so-called xe2x80x9cdual morexe2x80x9d oscillator. A dual mode oscillator includes a crystal that oscillates in two distinct modes. One mode is relatively frequency stable with respect to temperature, while the second mode is relatively temperature sensitive. The temperature sensitive mode is used to enable determining the temperature of the crystal, and that temperature determination is then used to adjust the frequency of the thermally stable mode for temperature variations. See for example, U.S. Pat. Nos. 5,525,936 and 4,079,280. To further improve the accuracy of a dual mode clock, one system includes having the clock in a temperature-controlled chamber. Such as system is disclosed in U.S. Pat. No. 5,004,987.
One application for highly temperature stable clocks is in well logging instruments and drilling instruments known collectively as measurement while drilling (MWD) systems. MWD systems known in the art include various types of sensors disposed in a drill collar. The drill collar is adapted to form part of a drilling tool assembly. The sensors in the MWD system make measurements related to various properties of earth formations surrounding a wellbore drilled through the earth by the drilling tool assembly, as well as various drilling parameter measurements. Typically, many of the measurements are recorded in some type of storage device disposed in the drill collar. An important part of the record of the measurements stored in the storage device is the precise time at which the measurement is made. The time record is typically correlated to a time/well depth record made at the earth""s surface so that the recorded measurements may be recharacterized with respect to depth in the earth at which the measurements were made. It is important that the time record made by the instrument in the wellbore be as time-accurate as possible to avoid subsequent depth correlation errors.
Other applications for highly temperature stable clocks in well logging instruments include operating a telemetry system such as used in wireline conveyed well logging instruments. Telemetry systems are used to communicate measurements made by the instrument to a recording and interpretation system disposed at the earth""s surface. See, for example, U.S. Pat. No. 5,504,479 issued to Doyle. As explained in that patent, typical well logging instruments are subjected to wide temperature variations during use, and as a result, the telemetry clock can change frequency.
Techniques known in the art for stabilizing clock frequency are not very practical for use in MWD or wireline logging instrument systems mainly because the instruments are subjected to wide temperature variations, and such temperature variations can take place over a relatively short period of time. Further, using an oven to control the temperature of the clock to a very high degree of precision has proven impractical for use in MWD and wireline well logging systems. What is needed, therefore, is a temperature stable clock for use in well logging systems that can maintain frequency stability under wide and/or rapid variations in ambient temperature.
One aspect of the invention is a clock system which includes a dual mode oscillator crystal having a first output having a frequency related to a temperature of the oscillator crystal and a second output having a frequency substantially stable with respect to temperature. The clock includes a temperature maintenance device. The temperature maintenance device and the oscillator crystal are disposed in a thermally insulated chamber. The clock includes a processor operatively coupled to the temperature maintenance device and the oscillator crystal. The processor is adapted to operate the temperature maintenance device so as to maintain a temperature of the chamber within a predetermined range. The processor is adapted to calculate a substantially constant frequency clock signal using the frequency of the second output and a ratio of the frequency of the first output with respect to the frequency of the second output.
In one embodiment, the processor is adapted to receive instructions characterizing a relationship of the frequency of the first output and the frequency of the second output with respect to a frequency of the second output over a selected temperature range. In another embodiment, the processor is adapted to operate the temperature maintenance device to cause the temperature of the oscillator crystal to pass through a selected operating range, and the processor is adapted to receive an external clock signal to characterize drift in a corrected frequency output of the clock with respect to the external clock signal and the temperature determined from measurement of the frequency of the first output and the frequency of the second output.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.